1. Field of the Invention
This invention relates to super hard inserts for drill bits and their assembly.
More particularly, this invention relates to polycrystalline compact cutter inserts for diamond drag bits. The compact cutters are mounted to diamond bit bodies and a means is provided to maintain each cutter in a sharp condition while performing in an earthen formation.
2. Background
Synthetic cutter inserts consist of a super hard face supported by a stud or cylindrical body usually fabricated from tungsten carbide. The tungsten carbide support substrate has, for example, a diamond layer sintered to the face of the substrate. The substrate is then typically brazed to a stud body. The insert is then pressed into an insert hole formed in the face of a drag bit or a cone of a rotary cone rock bit.
Alternatively, the diamond layer may be sintered directly to a cylindrical body that is subsequently brazed to the cutting face of a drag bit. The diamond layer is composed of a synthetic polycrystalline material. One such manufacturer of the super hard layer is Megadiamond, a division of Smith International, Inc. located in Provo, Utah.
One of the most common type of super hard insert is a polycrystalline diamond compact (PCD) cutter used in diamond drag bits for drilling, which is cylindrical. A cylinder type PCD consists of a right cylinder tungsten carbide body with a thin layer of polycrystalline diamond chemically and metalurgically bonded to an end face of the cylinder using a high pressure/high temperature (HP/HT) sintering process.
Typically, a cylinder type PCD insert cutter is fixedly mounted, by brazing, in a socket formed on the outer surface of a cutter blade fabricated on the drilling face of a drag bit. The PCD insert cutter is, for example, positioned with back rake and heel clearance for the diamond cutting face by tilting the trailing end of the cutter body upward in relation to the borehole bottom.
For drilling many ductile formations, presently utilized PDC cylindrical insert cutters are somewhat inhibited from aggressively cutting the formation by the tungsten carbide substrate immediately behind the layer of diamond. The carbide adjacent the PDC layer, being the same diameter as the sintered diamond, prevents the ultra hard cutter from maximum, penetration of the rock due to the penetration limiting effect of the carbide. Since tungsten carbide is softer than PDC, it will wear faster than the diamond. Experience has shown that, when the carbide wears and exposes the diamond face, the cutter becomes more aggressive.
Therefore, it has been determined that by providing a groove behind the PDC diamond layer nearest the portion of the cutter exposed to the formation, the insert cutter will more aggressively attack the formation resulting in greater rates of penetration.
A number of patents relate to methods and apparatus to relieve stress riser cracks formed at the braze junction between a tungsten carbide substrate supporting a diamond layer and a tungsten carbide stud or cylinder body.
U.S. Pat. No. 4,972,912 and 4,993,505 assigned to the same assignee as the present invention and incorporated by reference, teach methods to inhibit or prevent stress riser cracks at the aforementioned braze juncture.
U.S. Pat. No. 5,060,739 also teaches a means to prevent stress concentrations resulting from brazing a diamond layered disc to a tungsten carbide stud body.
The present invention provides a means to enhance the cutting ability of a super hard cutting face such as PDC layers sintered to a tungsten carbide substrate (studs or cylindrical bodies) for more aggressive performance of the cutter in an earthen formation.